In many communications systems, and more specifically, cellular radiotelephone systems, transmissions from a base-site to a mobile either has the capability to adjust transmission timing, or does not require any adjustment at all. However, in other cellular radiotelephone systems, specifically code-division multiple access (CDMA) cellular radiotelephone systems, absolute timing between the base-site and a subscriber unit is essential for effective operation of the system. The requirement for absolute synchronization is seen, not so much from a single base-site to subscriber unit perspective, but more from two or more base-sites to subscriber unit perspective. This is true since CDMA subscriber units are capable of diversity receiving transmissions from at least two base-sites, and making reception decisions upon either one, or both, of the transmissions as required. Clearly, to achieve this purpose, the two base-sites which perform the transmission of a signal must be synchronized absolutely in time so that the subscriber unit may receive both transmissions at the same time and perform reception as required.
One method of assuring synchronization between base-site in a CDMA cellular radiotelephone system is to use the global positioning system (GPS) which has been traditionally used for navigational purposes. GPS is a series of satellites synchronized in time and continuously transmitting, inter alia, time, date, and positioning information. By supplying GPS receivers at the base-sites, absolute synchronization of the communication system can be achieved. However, several inherent problems with GPS may occur. First, GPS may temporarily go out of service as the cesium or rubidium standards within the satellites require maintenance. Second, not every country in the world can receive a transmission from a GPS satellite.
To circumvent the inherent problems of GPS, co-located standards such as rubidium, cesium, or ovenized oscillators may be installed at the base-sites for backup purposes. However, as communication systems grow, the effect of having a GPS receiver and a standard for GPS backup in each base-site can become very expensive when analyzed on a per-channel basis for small and medium cell-sites. In fact, the standard and its related circuitry is one of the major costs for the base-site during both installation and maintenance of the system.
Thus, a need exists for a GPS backup configuration for use in a communication system that is centrally located so as to decrease the per channel cost of the standard while effectively maintaining absolute base-site synchronization in the communication system.